Her main efforts of the PhD research activity are devoted to the complete development of a new kind of semiconductor detectors for high energy resolution X-ray spectroscopy retaining the position information of the incident X-rays.
The new detector,
named Controlled Drift Detector (CDD) is almost a hybrid between two well
known detectors: i) Fully Depleted pn-Charge Coupled Devices and
ii) Silicon Drift Detectors.
It combines the
positive features of each detector without their drawbacks. It retains
the readout speed of the Silicon Drift Detector without the requirement
of the independent knowledge of the time of the X-ray conversion and the
pixel structure typical of the Fully Depleted pn-Charge Coupled
Devices with much faster transport of the signal charge. Moreover it retains
the very low capacitance of the charge collecting anode that allows precise
charge and hence energy measurement typical of both Fully Depleted pn-Charge
Coupled Devices and Silicon Drift Detectors.
The CDD is operated in integrate-readout mode thanks to the possibility of externally switchable longitudinal channel stops. When longitudinal channel stops are switched on, the integration phase takes place and the detector accumulates charges in distinctive pixels. When the longitudinal channel stops are switched off, the detector becomes the Silicon Drift Detector with confined lateral diffusion. Charge accumulated during the integration phase is left to drift towards the anode with the velocity typical of a Silicon Drift Detector. The drift time, that is, the time between the moment the channel stops were removed and the arrival time of electrons to the anode defines the position of the pixel from which the charge was released. Three different schemes to switch the longitudinal channel stops were developed.
From the very beginning she cooperated in the proposal of the new detector. She analysed and designed the different implementations producing the layouts for the different detectors. The designed detectors were produced at the MPI Halbleiterlabor in Munich, Germany. She took care of the development of the experimental apparatus for the detectors characterization. During the last two years she characterised two of the designed prototypes solving the unforeseen problems and difficulties. Both the prototypes are properly functioning. The measured readout times are less than 3ms for a 1cm long detector, well below the time required to readout a Fully Depleted pn-Charge Coupled Devices for spectroscopic applications.
A patent about the CDD concept has been deposited in the European Countries and in the USA.
She collaborated to the development of a Silicon Drift Detector with a spiral-shaped electron trajectory for 2-D position measurements.
more about the Spiral Drift Detector
The solution is to use for the reset path an active device (pnp bipolar transistor or p- MOSFET). The comparison of the performances achievable with the two devices shows that the best solution in terms of added noise and of linearity of the response would be a bipolar transistor for the values of leakage currents typical for semiconductor detectors for high resolution spectroscopy. However, the available technological process and the layout compatibility have driven towards a p-MOSFET operate in the sub threshold mode.
She designed the p-MOS transistor embedded in the front-end JFET taking care of both the simulation and the design of the structure. Many problems had to be solved due to the reduced space for the integration of the MOSFET and to keep the JFET properly operating even with the introduction of the reset device. At present the layouts are ready to be delivered to the MPI Halbleiterlabor in Munich, Germany, where this detectors will be produced. Moreover, she is designed the transresistance amplifier in BiCMOS technology to be coupled to the on-detector-chip structure. Many problems arise in the amplifier design to cope with the particular characteristics of the front-end transistor of lower transconductance than commercially available JFETs. The requirements in terms of noise and bandwidth are very demanding to cope with the design criteria of this electronics. At present the transresistance amplifier is under test.
The method is based on a physical model of the interface that allows to correctly approximate the boundary condition in the interface region. The model assumes that the interface region is divided in two regions, an equipotential region that corresponds to the region where the electron accumulate and a fully depleted region. The extension and the potential of the electron layer are calculated with the desired precision by an iterative procedure.
This method has been implemented in a 3D Poisson solver previously developed in the group of Prof. Gatti. The main advantage of the proposed method is to properly simulate semiconductor detectors that operate in conditions of full depletion by solving only the Poisson equation. She directly validate the proposed method by comparing the results achievable with the proposed simulator with a conventional two dimensional drift-diffusion simulator. this simulator has shown very useful in the study of the potential profile within the volume of a semiconductor detector in different biasing conditions at low CPU time.
The method have been fully developed, implemented on a personal computer and applied to study the effects of the multiple readout on the different noise contributions. She studied also the effects of the leakage current thermally generated on the performances achievable with this readout technique.
Moreover, she designed a new device to embody the multiple readout in X-ray detectors based on a high resistivity substrate, like the fully depleted pn-Charge Coupled Device, the Controlled-Drift Detector, the Silicon Drift Detector. At present the device is under test.
E-mail: Chiara.Guazzoni@mi.infn.it
Phone: +39-02-2399.6147 Lab: +39-02-2399.6152
+39-02-2399.6312 Fax: +39-02-2367604